Method of electrostatically coating recessed surfaces



' Sept. 17, 1968 D. M. FREDRICRSON 3,401,628

METHOD OF ELECTROSTATICALLY COATING RECESSED SURFACES Filed July 2, 1964 POTENTIAL SOURCE POTENTIAL SOURCE q. I I I I I I |I I II a h I n I I Q r m MEI-1...! N L I77/// /////////u V77 l /A// Q K m 5: &

SOURCE POTENTIAL POTENTIAL SOURCE IONIZED FLUID INVENTOR. DONALD MARVIN FREDRI CKSON ATTORNEY POTENTIAL SOURCE United States Patent 3,401,628 lviETHQ-D 0E ELECTROSTATICALLY COATING RECESSED SURFACES Donald Marvin Fredriclrson, Trenton, N.J., assignor to American Can Company, New York, N.Y., a corporation of New Jersey 2, 1964, Ser. No. 379,970

Filed July 4 Claims. (Cl. 101-129) AEESTRACT GT3 THE DISCLOSURE This invention relates to a method for coating the recesses on the surface of a material, and more particularly to a method of utilizing an electrostatic printing technique to coat the desired depressions on an irregular surface without depositing coating upon other areas of the surface.

It is a familiar technique to decorate surfaces having depressions therein by coating these recesses and leaving the generally planar outer surface uncoated. The usual method for doing this is to coat the entire surface of the article, including both the recesses and the non-recessed areas of the material. Thereafter the excess coating material that has been deposited on those areas not recessed is generally removed by wiping. The remaining paint within the recesses delineates the design that is desired to be emphasized and is not generally subject to excessive abrasion through handling.

It is quite obvious that this method of decorating is both expensive and wasteful. More material must be utilized than is needed for the decorating operation and in addition the procedure of removing this excess material is both time consuming and costly.

It is therefore an object of the present invention to provide a method for coating recesses on the surface of an irregular material without coating the non-recessed areas of the material.

It is another object of the present invention to provide a method of coating recesses upon the surface of an irregular material without directly contacting the surface of the material.

Still a further object of the present invention is to provide a method of coating the desired recessed portions of a generally planar surface without also depositing the coating on the surface of other areas of the material.

Yet another object of the present invention is to provide an inexpensive and simple method of decorating an article having an irregular surface.

A still further object is to uniformly coat the recesses within the surface of a non-planar material.

Numerous other objects and advantages of the invention will be apparent as it is better understood from the accompanying description, which, taken in connection with the accompanying drawings, discloses a preferred embodiment thereof.

The above objects are accomplished by positioning a material which is to be decorated between an electrically conductive screen having a plurality of apertures therein and spaced therefrom and an electrically conductive substance. The apertures are aligned with recesses within said material that are to be coated and the screen is spaced from the material. The screen and the electrically conductive substance are connected in an electrical circuit and Patented Sept. 17, 1968 ice an electrical potential difference is established between the screen and the substance. Finely divided pigment particles are brought to the outer surface of the screen. These pigment particles are of a size less than the size of the screen apertures and are thus able to pass through these apertures. The particles are then passed through the apertures and into an electrostatic field created between the conductive screen and the conductive substance when the electrical potential difference is established. Upon entering this electrostatic field the particles will be forced towards the material and will be deposited within the recesses that are in alignment with the screen apertures. Thereafter the deposited particles are coalesced to form a continuous coating within the recesses.

Referring to the drawings:

FiGURE 1 is a perspective view of an electrostatic coating apparatus for carrying out this invention;

FIG. 2 is a sectional view taken substantially along the line 2-2 in FIG. 1;

FIG. 3 is an enlarged partial sectional view illustrating the method of coating a recess in an embossed sheet;

FIG. 4 is an enlarged partial sectional view showing a modified form of coating a recess within an embossed sheet;

FIG. 5 is a sectional view showing the apparatus for coating recesses within the outer surface of a container.

As a preferred or exemplary embodiment of the instant invention, FIGS. 1 and 2 illustrate an embossed sheet 10 positioned between a conductive screen or stencil I2 and a conductive substance 14. As shown in the drawings the conductive substance 14 is a metallic plate.

Although in the instant embodiment, the embossed sheet 10 is made of a plastic material, it is to be understood that this sheet may be made of either conductive or non-conductive materials such as fiber, metal, etc. If the sheet 10 is made of a conductive material, it may be used as both the conductive substance and as the material to be decorated. A series of recesses 16 and 18 are embossed in this sheet by means well known to those skilled in the art. Although the recesses are shown to be rectangular and circular in shape, it is readily apparent that the shape may be of any type desired.

Within the stencil 12 are a plurality of open screen areas 20, 22 which conform in shape to the recesses 16 and 18 respectively. The remainder of the stencil is covered by a suitable masking material or, if desired, a separate masking plate or template 23 could be utilized to define the screened areas through which electrostatic printing will take place. Although the apertures 24 in the screen 12 may vary in size as desired, the usual procedure is to use a screen of from to 300 mesh.

One pole of a direct current potential source is connected to the conductive stencil 12 while the other pole of the potential source is connected to the conductive plate 14. When current is induced into the circuit an electric field is established in the space between the stencil I2 and the plate 14 due to the potential difference thereetween.

Finely divided pigment particles 25 are brought into contact with the outer surface of the stencil 12 by suitable means, such as the illustrated roller 26 onto which has been placed a quantity of particles. Although the roller means is shown as the method of feeding the particles to the stencil 12, it is readily apparent that other mechanical means such as brushes, or air jets may be utilized. In addition, the particles may be transported to the screen 12 by means of electrical charges.

In carrying out this invention, the sheet 10 is placed between the stencil 12 and the plate 14 so that sheet 10 is spaced from the stencil 12, leaving an air gap 27 therebetween. Although the plate 14 is shown to be also spaced from the back portion of the sheet 10, this is 3 not a requirement but may be done for convenience. If desired, the plate 14 may abut the back of the sheet The conductive stencil 12 and the conductive plate 14 are then connected to a suitable potential source.

When the screen areas 20, 22 are aligned with their corresponding recesses 16, 18 in the sheet 10 the coating operation is then ready to begin (FIG. 3). The roller 26 has previously been covered with a suitable quantity of coating material, in the form of fine powder particles 25. As the roller 26 is moved across the outer surface of the stencil 12, the particles 25 adhering to the roller are formed through the apertures 24 in the screens 20, 22.

The powder particles 25 assume the charge of the conductive stencil screen 12 by contact. In addition, the particles 25 become triboelectrically charged through contact with the feed roller or brush. As there is a potential difference between the stencil 12 and the plate 14, the particles 25 passing through the apertures 24 move through the electrostatic field in the air gap 27 toward.

the plate 14. However, since the sheet 10 lies between the stencil 12 and the plate 14, the particles 25 are deposited upon the sheet 10 in a pattern conforming to the screen areas 20, 22 within the stencil 12. As the recesses 16, 18 are aligned with these screen areas 22, respectively, the particles are thereby deposited only within the recesses and not upon the generally planar surface areas of the remainder of the sheet. Thus a coating 28 will be formed within the recesses 16, 18 and will continue to build-up as long as particles are passed through the respective screen areas aligned with the recesses and the potential difference is maintained.

Since the apertures 24 are of substantially uniform size, the thickness of the coating 28 upon the surface 30 of the recesses will vary depending upon the angle the surface 30 maintains with the plane of the screen area. Those areas of the surface 30 which are substantially parallel to the plane of the screen area will have a thickness somewhat different from those areas of the recess which are more normal to the plane of the screen area. This is due to the path the particles 25 travel between the screen 12 and the sheet 10 and the surface areas to be coated, the path being essentially normal to the plane of the screen areas 20, 22.

In a modified form of the invention, as shown in FIG. 4, the size of the apertures 24 will be varied somewhat in order to produce a more uniform thickness of the coating 28 on the surface 30 of a recess. In this form of the invention, the electrostatic coating technique will be essentially the same as that described hereinbefore. However, since the size of the apertures 24 near the edges 32 of the screen areas 20, 22 will be somewhat larger than those towards the center 34, more pigment particles 25 will be permitted to pass through the apertures 24 at the edges 32 of the screen areas 20, 22, than will pass through the center portions of the screen areas 20, 22. This will result in a tendency for a more uniform thickness of the coating 28 to be formed upon the surface 30 of the recesses aligned with their respective screen areas.

After the coating 28 has been deposited within the recesses 16, 18 it is necessary for this coating to be coalesced in order to securely adhere it to the surfaces of the recesses. It is readily apparent that the procedure for coalescing will vary according to the type of particle used for the coating. Two methods which are apparent to those skilled in the art are heat fusing or solvent solution.

Since there are many variables present in the process, such as distance between sheet 10 and the stencil 12; the size of the powder particles; the magnitude of the DC. potential; etc., these conditions may be varied in carrying out this invention without in any way departing from it.

Although the air gap 27 between the conductive screen 12 and the sheet 10 is preferably as narrow as possible,

in order to reduce the magnitude of the voltage necessary to effect transfer of the particles 25, it has been found that this gap may be as great as 0.50 inch. It is apparent that the greater the magnitude of the gap 27 the greater will be the magnitude of the potential difference between the screen 12 and the conductive substance 14.

Particle size will, of course, vary with the size of the screen apertures. Certain powders have been found to be more responsive to certain polarities than others and will, therefore, produce better results for a given polarity of the conductive screen. However, any suitable pigment powder may be utilized in carrying out this invention, so long as the particle size will not be so great as to prevent its passage to the screen apertures.

The magnitude of the potential difference between the stencil 12 and the plate 14 will be such as to provide a satisfactory movement of the particles 25 toward the screen 10 without resulting in an electrical short between the stencil 12 and the plate 14. Voltages between 500 and 10,000 volts DC. are generally adequate for carrying out this process.

Although the invention has been described with the use of generally planar sheets of recessed material, it is readily apparent that other shapes having recesses therein may also be coated using the process of this invention. For instance, ordinary glass or thermoplastic bottles having depressions in their outer surfaces may be coated using the process hereinbefore described.

If an article, such as -a bottle 40 is to be coated (see FIG. 5), another type of conductive substance rather than the plate 14, hereinbefore described, may be used. A fluid substance such as a conductive liquid or an ionizable gas 42 may be placed within the bottle 40, an electrode 44 may then be placed within the neck 46 of the bottle and a suitable voltage impressed thereon to charge the liquid or ionize the air or other gas within the bottle 40 thereby providing a conductive fluid substance to which a conductive screen 12a may be electrically connected. Thus the electrostatic field for carrying out the transfer of the particles from a source, such as the roller 26a, through the conductive screen to the object to be coated is created.

The term conductive substance, as used herein, is intended to mean a substance that will cooperate with an electrode when an electrical potential is applied thereto, to establish an electric field therebetween which can transfer charged particles as described herein.

In some modifications of this concept it is possible that the image bearing screen may not itself be charged, but will serve only to define an image area for particles delivered from a charged plane above it. For instance, the roller or brush applicator may be the charged plane.

It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the steps of the methods described and their order of accomplishment without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred embodiment thereof.

I claim:

1. A method of substantially uniformly coating recesses upon the surface of container wherein portions of said recesses are substantially parallel to an electrically conductive screen and portions of said recesses are substantially normal to said screen, comprising the steps of:

positioning said container adjacent to, but spaced from,

an electrically conductive screen having a plurailty of varied-sized, relatively large apertures to be placed in alignment with said substantially normal recessed portions and relatively smaller apertures to be placed in alignment with said substantially parallel recessed portions therein, said container having therein an electrically conductive ionizable fluid, said screen and said fluid being connected in an electrical circuit; aligning said screen apertures with said recesses and spacing said screen from said material so that the larger apertures are aligned with those portions of said recesses being most normal to said screen and the smaller apertures aligned with those portions of said recesses being most parallel to said screen; establishing an electrical potential difference between said conductive screen and said conductive fluid to ionize said fluid and establish an electrostatic field between said screen and said ionized fluid; bringing finely divided pigment particles to the outside surface of said screen, said particles having a size less than the size of the smallest of said screen apertures and able to pass through said apertures; passing said particles through said apertures and into said electrostatic field between said conductive screen and said ionized fluid, whereby said particles will be forced toward said container and will be deposited in a substantially uniform coating upon the surfaces within said recesses in alignment with apertures;

and coalescing said deposited particles to form a continuous coating within said recesses.

2. The method of claim 1 wherein said finely divided pigment particles are triboelectrically charged.

3. The method of claim 1 wherein in the step of establishing an electrical potential difference, said electrical potential difference is between 500 and 10,000 volts DC 4. The method of claim 1 wherein said deposited pigment particles are coalesced by heating.

said screen References Cited UNITED STATES PATENTS 2,024,087 12/ 1935 Ballard 10l--127 2,341,827 2/ 1944 Sukumlyn 118-301 XR 3,191,574 6/1965 Robertson 118-308 XR ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Examiner. 

